path.c: translate Windows paths recorded by Windows git on POSIX hosts

[johnnyshields]

The Problem

Worktree gitdir paths do not work when a filesystem is shared between WSL Linux and Windows.

If I create a git worktree using Windows Git, then git inside WSL Linux cannot resolve its path, because the gitdir specifier will be a Windows-style path C:\repo\... rather than a POSIX-style /mnt/c/repo...

The vice-versa issue also applies, Windows Git can't read gitdir from a worktree in created in WSL.

As background, I use a hybrid Windows and WSL Ubuntu setup on the same machine; it works nearly perfectly except for the above issue, specifically with worktrees.

Root Cause

Git worktrees use a bi-directional link that include absolute paths in the gitdir value to point from the main repo to the worktree repo, and vice versa. It is specifically this gitdir value that needs Windows-to-POSIX path translation handling. (Nothing else besides this path requires additional logic for worktrees to work across WSL and Windows native.)

  <main-repo>/   ← e.g. C:\workspace\myrepo (or /mnt/c/workspace/myrepo from WSL)
  └── .git/
      └── worktrees/
          └── <name>/
              ├── gitdir      ← file. content: "<worktree>/.git\n"
              ├── commondir   ← file (sometimes). content: relative or absolute path back to main .git
              ├── HEAD
              ├── index
              └── config.worktree

  <worktree>/    ← e.g. C:\workspace\myrepo-wt
  └── .git       ← FILE (not a dir). content: "gitdir: <main-repo>/.git/worktrees/<name>\n"

When git worktree add is run from native Windows, git writes absolute paths into the worktree's .git file, into <commondir>/worktrees/<id>/gitdir, and (when present) into <commondir>/commondir, in <x>:/... or <x>:\... form. Reading those files back from a non-Windows-native build of git fails because neither form is meaningful on POSIX, so the worktree appears broken even though every byte of it is reachable - the most common scenario being a worktree on a Windows drive opened from inside WSL2 (where the Windows filesystem is mounted at /mnt/<x>/) or from Cygwin/MSYS (where it is /cygdrive/<x>/).

The Solution (What this Patch Does)

This patch adds a small helper translate_windows_path() that recognizes this shape at the start of a path.

• If GIT_WINDOWS_NATIVE is defined, this helper rewrites it to the Windows path <x>:/.
• Otherwise, if GIT_WINDOWS_NATIVE is not defined, this helper rewrites it to the POSIX mount form appropriate for the current build (/cygdrive/<x>/ on Cygwin, /<x> on MSYS2, /mnt/<x>/ everywhere else), converting backslashes to forward slashes.

This helper is called everywhere git reads a recorded worktree-related path back from disk:

• read_gitfile_gently() - the gitdir: line in a worktree's .git file.
• get_common_dir_noenv() - the commondir file inside a worktree's git directory, which points at the main repo.
• get_linked_worktree() - the gitdir file inside <commondir>/worktrees/<id>/, which points at the worktree's .git link.

Translation only happens for <x>:/ or <x>:\ where <x> is a single ASCII letter; anything else is left alone. The helper is a no-op on GIT_WINDOWS_NATIVE builds, where the input is already in native form. On non-WSL Linux hosts the translation still produces a syntactically valid POSIX path; if the corresponding /mnt/<x>/ mount does not exist, the next stat()/open() fails as it would have without translation - i.e. the change cannot make a working configuration stop working.

Why this is Safe

This PR is safe because:

• There is no effect/change for users not using a Windows-POSIX shared filesystem setup. Specifically there's no change to how POSIX git reads POSIX paths, and no change to how Windows git reads Windows paths. The only change is that POSIX can now interpret Windows-style paths and vice-versa.
• The logic is read-only: it only affects how Windows-style paths are read from POSIX; it does NOT mutate how the paths are stored in .git's definitions.
• Path traversal safety: the path translation only happens for the left-anchored part of the string <x>:\ where <x> is a single ASCII letter (e.g. C:\). Anything else (e.g. http://) is left alone. The helper also doesn't evaluate/collapse relative paths, so it doesn't introduce any path-traversal vectors that aren't existing today.

Known Limitations / Caveats

TLDR; this PR is targeted to solve the pain point as it affects the 99% majority of "happy-path" Windows-WSL shared filesystem users. It is intended to gracefully degrade in the 1% of non-happy-path edge cases.

1. This PR assumes that POSIX users are using /mnt/<x> as their Linux mountpoint for Windows drives, with /mnt/ being hardcoded. This is a reasonable assumption because (1) every currently available WSL Linux distro uses /mnt/ by default (even non-strict-FHS distros like NixOs), and (2) supposing a user renames their /mnt/ mountpoint to something else, Git will simply gracefully degrade to not detect the cross-OS filesystem path. I don't think there's a real need to support this edgecase, but if we are seriously concerned about it we could expose a git config such as wslMountPath = /my/path
2. Windows will detect MSYS's /c/ as C:\. Technically this could clash with /c/ being a path on Linux, but the odds of this happening in practice are extremely remote: the user would have to (A) be using git on Windows, and (B) have a worktree in a Windows visible path, and (C) have it's parent repo in a completely separate Windows-invisible path, and (D) having an identical clashing path in your Windows filesystem.
3. I've used an #ifdef to change the guts of the translate_windows_path() for POSIX vs. non-POSIX. Conceptually I think this works in this case, as it minimizes the number of call sites (essentially the method is "do the right thing depending on OS") but if this way of coding is not kosher for the project standards I am happy to split it into translate_windows_to_posix_path() and translate_posix_to_windows_path() functions. It will mean more #ifdefs are added downstream.

Should this be a PR for "Git on Windows" instead?

I think no, because in order to support reading Windows paths from WSL (Windows-to-POSIX) we need to apply a patch to Linux Git. Since the equivalent POSIX-to-Windows patch lives in the exact same place in the code, it's best to just do them both together to avoid conflicts.

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